WorksheetsApply What You Have Learned
Total questions: 25
Worksheet time: 13mins
Botulinum neurotoxin blocks acetylcholine exocytosis at the neuromuscular junction. Which outcome most directly results from this blockade?
Rapid ATP hydrolysis by detached myosin heads
Hyperactive crossbridge cycling within myofibrils
Excessive calcium release from the sarcoplasmic reticulum
No end-plate potential is generated in the muscle
During crossbridge cycling, ATP becomes unavailable while sarcoplasmic calcium concentration stays high. Select all outcomes that would occur.
The muscle fiber is unable to relax effectively
Troponin detaches calcium, exposing fewer binding sites
Acetylcholine release increases to compensate
Myosin heads remain tightly bound to actin filaments
Identify the structure that invaginates the sarcolemma to transmit action potentials deep into the muscle fiber, as shown in the diagram.
SR terminal cistern
Synaptic knob
Myofibril
T-tubule
Which labeled feature stores Ca2+ and releases it to trigger contraction in skeletal muscle?
Ligand-gated channel
Z disc
SR terminal cistern
Motor end plate
At the neuromuscular junction, which event directly follows acetylcholine binding to its receptor in the motor end plate?
ATP synthesis by mitochondria
End-plate potential generation
Thick filament detachment
Calcium uptake into SR
Select the components that together form a triad in skeletal muscle fibers.
Two motor end plates plus one T-tubule
One sarcolemma plus one synaptic knob
Two T-tubules plus one myofibril
One T-tubule plus two SR terminal cisterns
Which statement best distinguishes thick from thin filaments in the sarcomere diagram?
Thick filaments contain myosin heads
Thin filaments anchor at M line
Thick filaments anchor at Z discs
Thin filaments contain myosin heads
In the sarcomere image, the Z disc serves primarily to anchor which filaments?
SR cisterns
T-tubules
Thick filaments
Thin filaments
Which labeled region represents the central line where thick filaments are linked and cross the sarcomere midline?
Synaptic cleft
Z disc
M line
Motor end plate
A toxin blocks exocytosis of acetylcholine at the synaptic knob. Based on the diagrammed junction, which outcome is most immediate?
Excess Ca2+ release from SR
No end-plate potential forms
Sarcomere shortening increases
Rapid crossbridge detachment
Which event first triggers acetylcholine release at the neuromuscular junction?
End-plate potential at the sarcolemma
Action potential reaching the synaptic knob
Calcium binding to troponin on actin
ATP hydrolysis on myosin heads
What directly causes synaptic vesicles to exocytose acetylcholine?
Opening of ligand-gated ACh receptors
Depolarization of the T-tubule membrane
Ca2+ entry into the synaptic knob
Na+ influx into the muscle fiber
Binding of acetylcholine to its receptor on the muscle fiber results in which immediate effect?
Release of myosin from actin
Power stroke of myosin heads
Opening voltage-gated Ca2+ channels in SR
Generation of an end-plate potential
Which structure propagates the muscle action potential deep into the fiber?
Sarcoplasmic reticulum network
Myofibril thick filaments
Transverse (T) tubule system
Z-disc anchoring proteins
At the triad, what initiates opening of calcium channels in the sarcoplasmic reticulum?
Na+ diffusion into the sarcoplasm
Troponin conformational change
ATP binding to myosin heads
Voltage-sensitive receptors activation
Which statement best describes the role of troponin and tropomyosin in resting muscle?
Block myosin binding sites on actin
Anchor thick filaments to M-line
Generate the end-plate potential
Hydrolyze ATP to energize myosin
Calcium released from the sarcoplasmic reticulum binds to which protein to expose myosin-binding sites?
Myosin regulatory light chain
Dihydropyridine receptor
Troponin on the thin filament
Tropomyosin head domain
Which sequence correctly describes the early steps of excitation of a skeletal muscle fiber?
Motor neuron AP reaches axon terminal
Ca2+ enters synaptic knob to trigger exocytosis
ACh binds ligand-gated channels on sarcolemma
ATP hydrolysis energizes myosin heads
During the power stroke, what mechanical action occurs?
Troponin binds additional Ca2+ ions
Actin detaches and slides outward
Myosin pivots, pulling actin inward
ATP is synthesized by myosin heads
Which event allows myosin to detach from actin after a power stroke?
Depolarization of the sarcolemma
Further opening of ACh receptors
Binding of another ATP to myosin
Loss of calcium from troponin
What condition is required for crossbridge cycling to continue?
Continuous acetylcholine synthesis
Presence of calcium and ATP
Closed voltage-gated channels
Elevated sodium outside the fiber
End-plate potential has which immediate consequence near the NMJ?
Opening of SR ryanodine channels directly
Detachment of myosin from actin
Release of tropomyosin from actin
Threshold for muscle action potential
Which pairing is correct in excitation–contraction coupling?
Na+ binds troponin to start contraction
Ca2+ binding exposes actin sites
SR releases Ca2+ into sarcoplasm
T-tubule depolarization activates voltage sensors
What happens to sarcomere length during repeated crossbridge cycles?
Thin filaments slide toward center
Thick filaments move toward Z-disc
Length remains unchanged overall
Z-discs drift away from each other
Which event is NOT part of the initial synaptic transmission at the NMJ?
Voltage-gated Ca2+ channels open
ACh diffuses across synaptic cleft
ACh binds ligand-gated receptors
ATP hydrolysis by myosin heads
